Intrinsically conducting polymers are of wide utility in applications such as electronic packaging, organic light-emitting diodes (LEDs), electrochromic windows, volatile organic gas sensors, and the like. Intrinsically conducting polymers of particular interest possess a relatively low band gap (Eg), which refers to the energy difference between two electronic energy levels (the conduction band and the valence band). The band gap exhibited by a given polymer depends upon a variety of factors, including the structure of the monomer(s) used to form the polymer. For example, intrinsically conductive polymers formed from thiophene and substituted thiophene monomers are known. Poly(thiophene) has a band gap of 2.1 electron volts (eV), poly(2-decylthieno[3,4-b]thiophene) has a band gap of 0.92 eV, and poly(2-phenylthieno[3,4-b]thiophene) has a band gap of 0.85 eV. Intrinsically conductive polymers comprising polymerized units of thieno[2,3-b]thiophene and thieno[3,2-b]thiophene are also known.
Unfortunately, there are a number of drawbacks associated with many of the intrinsically conducting polymers currently available. The Eg of many polymers is undesirably high, and/or the polymers are not stable. Transparency is difficult to achieve, limiting their use in optical devices. Those consisting solely of aromatic repeat units in the backbone are typically not soluble in water, requiring manufacture and processing in organic solvents.
Of course, replacement of organic solvents with aqueous solvents in the synthesis, use, and processing of intrinsically conductive polymers would be highly desirable from an environmental and cost standpoint. U.S. Pat. No. 5,300,575 discloses dispersions of poly(thiophenes) that are suitable for use as antistatic coatings for plastic moldings. The poly(thiophenes) are prepared by polymerizing a di-substituted thiophene (e.g., 3,4-ethylenedioxythiophene) in the presence of a polyanion and oxidizing agents, oxygen and/or air. The resulting linear polymer is purified using both anion and cation exchange resins wherein poly(styrene sulfonate) serves as a charge compensating dopant. The resulting polymer forms a colloidal dispersion in water, presumably because poly(styrene sulfonate) is soluble in water and demonstrates a strong ionic interaction with the cationic poly(3,4-ethylenedioxy)thiophene backbone. The resulting polythiophenes have an Eg of 1.7 eV.
There remains a continuing need in the art, however, for intrinsically conducting polymers that exhibit useful band gaps for industrial applications, that can be readily dispersed or dissolved in water, and for the convenient adjustment of the conductivity and/or optoelectronic properties such as the band gap and energy levels of the valence band and the conduction band of such polymers to meet the needs of a particular application.